This invention relates to preparation of polymers having copolymerized units of 2-chloro-1,3-butadiene and alkyl, hydroxyalkyl, or cycloalkyl esters of unsaturated monocarboxylic acids.
Graft copolymers of 2-chloro-1,3-butadiene (chloroprene) and alkyl acrylates or alkyl methacrylates have found utility in a variety of solvent and latex adhesive applications, particularly in the manufacture of shoes. Such copolymer compositions form strong bonds to polyvinyl chloride, polyurethane, and nylon substrates. In addition, they are unusually resistant to certain plasticizers commonly used in the footwear industry.
The copolymers may be prepared by a variety of methods, but, generally, industrial scale preparation consists of graft polymerization in a high temperature bulk solution process. That is, a polychloroprene base polymer is dissolved in an organic solvent and alkyl acrylate or alkyl methacrylate monomers are introduced in the presence of a free radical initiator at temperatures above 80.degree. C. Other methods have also been disclosed in the prior art, for example, in U.S. Pat. No. 3,728,316; Japanese Patent Application Kokai 61-275315; and Japanese Patent Application Kokai 1-284544, wherein aqueous emulsion processes are utilized to prepare chloroprene/alkyl acrylate or chloroprene/alkyl methacrylate graft copolymers.
Whether prepared by solution or emulsion processes, the resultant products have backbones composed of copolymerized units of chloroprene and alkyl acrylates or alkyl methacrylates. In addition, the copolymers contain grafted side-chains derived from the alkyl acrylate or alkyl methacrylate.
Due to environmental and health concerns, the use of solvent processes is increasingly disfavored. Therefore, it would be desirable to have available an efficient emulsion process for preparation of chloroprene/alkyl acrylate and chloroprene/alkyl methacrylate graft copolymers. Unfortunately, the emulsion processes of the prior art are either inherently inefficient or suffer from other significant disadvantages. For example, the preferred method for preparation of graff polymers would be a single stage process. However, in order to realize the efficiencies necessary for industrial scale production in single-stage reactions of the prior art, reaction temperatures of at least about 35.degree. C. have been required. Use of such high temperatures has the undesirable effect of producing discolored products. The less preferred multi-stage emulsion copolymerization processes exhibit other disadvantages, even though it is possible to operate such processes at low or moderate temperatures in a first stage. In a typical multi-stage reaction, the first copolymerization stage is followed by removal of unreacted monomers. In a further step, conducted at a higher temperature, generally in excess of 80.degree. C., additional acrylate or methacrylate monomer is supplied to the reaction mixture resulting in grafting of acrylate or methacrylate monomer onto the copolymer backbone. Such multi-stage processes require long cycle times, complex reaction schemes, and complicated comonomer addition facilities. In addition, the emulsion is subjected to shear and thermal stresses resulting from transfer among vessels and heating in the intermediate phase wherein residual monomer is removed. This combination of shear and thermal stress results in polymer discoloration and degradation of polymer properties.
The present invention overcomes these problems by providing an improved single-stage reaction which produces non-discolored chloroprene/ester graft copolymers, such as chloroprene/alkyl acrylate or chloroprene/alkyl methacrylate graft copolymers in a highly efficient manner. By employing a particular method of temperature regulation in conjunction with control of monomer conversion, non-discolored products are produced without the disadvantages associated with a multi-stage process.